It is known that the petroleum industry routinely employs hydrocarbon conversion processes, and particularly cracking processes, in which hydrocarbon molecules having a high molecular weight and a high boiling point are broken down into smaller molecules having a lower boiling point which are suitable for use.
Many of these processes make use of fluidized-bed conversion techniques. In these fluidized bed techniques, solid particles supply the heat necessary for the conversion reaction. The particles are contacted with hydrocarbons for a very short time. The particles can be catalytic.
The process most widely employed at present is the so-called Fluid Catalytic Cracking, or FCC, process. However, other fluidized-bed conversion processes, such as thermal cracking or visbreaking processes, have also been developed.
For the sake of simplicity, the description which follows will be confined to the presentation of the invention within the framework of the catalytic cracking process, it being understood that the invention is applicable to most fluidized-bed hydrocarbon conversion processes in which the feedstock to be cracked is contacted in the vapor phase with solid particles, whether catalytic or not.
Among the most important parameters which determine the efficiency of a cracking reaction are the rapidity with which the feedstock to be treated is contacted with the hot catalyst particles and the homogeneity of distribution of these catalyst particles in the fluidized bed throughout the reaction zone.
The research conducted by Applicants' Assignee with a view to improving the heat transfer between the solid particles in the fluidized bed and the feedstock to be treated has shown that the yields actually obtained in the highest-efficiency cracking units in use up to now are below those to be expected on the basis of theoretical studies, and that this shortfall is due in particular to poor distribution of the catalyst particles in the reaction zone, and especially in the zone where the feedstock to be treated is injected.
In its French and U.S. patent applications, French Nos. 2,585,030, and 89 14787 (and the equivalent U.S. Ser. No. 07/612,322, filed Nov. 13, 1990 pending, incorporated herein by reference), Applicants and the Applicants' Assignee have already proposed means designed to remedy, within the reactor, the axial irregularities in the flow of hot catalyst coming from the regeneration zone and to provide for improved fluidization of the solid catalyst particles upstream of the zone where the hydrocarbons are injected.
However, even when the flow of catalyst is regularized so as to render it as homogeneous as possible upstream of the injection zone of the feedstock to be treated, it has been found that downstream of that zone the distribution of the catalyst particles again becomes heterogeneous, the density of distribution of these particles being higher in the vicinity of the walls of the reactor than in its center.
This natural tendency of the catalyst and the gas phase to segregate due to the interaction between the catalyst and the wall is intensified by the sudden vaporization of the feedstock, which tends to throw the catalyst toward the wall of the reactor, thus producing a concentration of catalyst in the vicinity of the reactor wall. A portion of the catalyst then advances only slowly or even tends to swirl in a direction counter to the motion of the feedstock (an effect known in the art as backmixing) in both an upflow and downflow fluidized beds.
The work carried out by the Applicants and their Assignee have shown that the spread observed between the theoretical yields and those actually obtained are also due in part to this poor distribution of the particles in the reaction zone after injection of the feedstock. This unequal distribution is attributable to the vortices produced by the combined effect of the sudden vaporization of the feedstock and of the high speed of injection thereof, as well as to the chafing of the catalyst particles against the walls. The result is what is known to those skilled in the art as backmixing, which also accounts for the fact that at the periphery of the reactor the catalyst particles are only slightly fluidized, if at all. Consequently, the particles, preferentially disposed at the periphery of the reactor, may stagnate or even flow back along the wall. As a result, the temperature distribution is not uniform throughout the section of the reaction zone downstream of the feedstock injectors. The temperature is excessively high at the periphery of the reactor since the particle density is too high near the walls. These excessively high temperatures cause the feedstock to be overcracked, interfere with the desired liquid conversion; and thus, promote the production of dry gases. Conversely, when the atomized feedstock comes into contact with a stream of catalyst particles that is not dense enough in the central part of the reactor, the quantity of heat supplied by these particles is not sufficient to raise the temperature of the feedstock to the level necessary for the desired reactions to take place; and thus, substantial coking of the catalyst occurs which leads to its deactivation.